The present invention relates to an RF probe for nuclear magnetic resonance imaging (MRI) devices.
Recently, an echo-planar method which provides superhigh speed image pickup has been studied in many makers and laboratories. A strong gradient magnetic field is required for achieving this method, so that a gradient magnetic field coil used in regular image pickup is not usable for achieving that method. Thus, a method has been proposed in which a strong small gradient magnetic field coil is set separately within a regular gradient magnetic field coil only when superhigh speed image pickup is made. An RF probe used only for superhigh speed image pickup is used further within the small gradient magnetic field in the MRI device of such construction.
RF probe used generally widely is a saddle coil, slotted tube resonator (STR), and a multiple element resonator (MER) which is also called birdcage resonator. QD (Quadrature Detection) method has widely been used to improve the sensitivity of reception and transmission efficiency of such coil. In this method, two orthogonal linear magnetic fields are generated whose phases are 90 degrees out of phase with each other such that a synthetic rotational magnetic field is generated or two received orthogonal linear magnetic fields are caused to deviate by 90 degrees out of phase with each other and then synthesized to detect a radio frequency rotational magnetic field with high sensitivity. Since reception and transmission are in electrically and electromagnetically reversible relationship, reception or transmission will be described below as required.
According to QD method, if the intensities of signals from two orthogonal coils are equal to each other, sensitivity is ideally improved by 1.4 times compared to single linear magnetic field detection. Another means for improving the detection sensitivity is a multiple coil technique, which is also called a phased array technique, used for surface coils. This is a technique where many relatively small-type surface coils which are of high sensitivity are situated on a subject to be detected to thereby increase a field of view while maintaining high sensitivity. In this case, it is required to sophisticatedly eliminate magnetic coupling and hence interference between the coils to prevent a decrease in the detection sensitivity which would otherwise occur due to such interference. Several such methods have been proposed. The QD method is disclosed, for example, in JP-A-4-17837. The MER is disclosed, for example, in U.S. Pat. No. 4,825,163, JP-A-60-132547 and -61-95234. The multiple coils as surface ones are disclosed, for example, in Publication of the translation of Internal Patent Application No. 1990/500,175, JP-B-2-13432, and JP-A-3-68342.
An RF probe with an RF shield is disclosed, for example, JP-A-58-39939. An example of an RF probe used with a small gradient magnetic field is disclosed in Radiology, vol. 181, pp.393-397.
Conventionally, the performance of those RF probes has not necessarily been optimized and they are not necessarily suitable for application to clinical diagnosis because their performance is not good and they are inconvenient to use.